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Reflect parentheses that discard extra values in the AST

See original GitHub issue

Currently, the latest released version does not distinguish e.g. f() from (f()) in the AST. These expressions are not equivalent; the former evaluates to all the return values of f(), and the latter evaluates to only the first of them (the rest are discarded). Current git master distinguishes them by the inParens field, but that is just a workaround for #24 and I expect to remove it when that issue is fixed properly.

A more principled approach to distinguishing these expressions is desired. I may keep the inParens field, or I may introduce a new kind of node that expresses the operation of discarding extra values.

Issue Analytics

State:
Created 4 years ago
Reactions:1
Comments:5 (1 by maintainers)

Top GitHub Comments

1reaction

fstirlitzcommented, Jan 16, 2020

There are two kinds of expressions which may evaluate to a value tuple: function calls and the vararg literal, .... There are also five places where discarding extra values matters, all of which are in a context where a multiple-value expression appears as the last one in a comma-delimited list of expressions:

table constructors ({ a, b, (...) }; table constructors can also use ; as delimiters, and accept trailing delimiters)
simultaneous assignment (a, b, c = d, e, (...))
argument lists (f(a, b, (...))
for-in loops (for a, b, c in d, e, (...) do --[[ ... ]] end)
return statement (return a, b, (...))

In the last of these, parenthesising the only expression also has the effect of preventing tail-call optimisation. This suggests a representation as a new type of node, so that tail calls can be detected simply by checking whether a function-call node appears as a direct child of the return node. This will be a breaking change for users who are not prepared to deal with unknown node types; luamin by @mathiasbynens would be one. In this case it’s arguably a good thing anyway, because as of now luamin will just perform minification incorrectly on code affected by this defect.

Alternatively, I might treat parentheses as a circumfix unary operator, and piggyback on the UnaryExpression node type to reflect it in the AST.

Lua manual links: 5.1 §2.5, 5.2 §3.4, 5.3 §3.4

0reactions

cohlercommented, Dec 20, 2022

This is a MAJOR BUG giving a totally useless and INCORRECT parse. Why hasn’t anyone fixed this yet?

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